Coupling of an electric motor or incandescent lamp with an ac source causes a surge current to arise which may be several- to ten-odd-fold larger than that observed when the electric motor or incandescent lamp is stationarily operated.
The present inventor disclosed in Japan Patent Kokai No. 189,727/84 that such surge current can be effectively suppressed by coupling an electric motor or incandescent lamp with an ac source through a phase-controlled automatic switching circuit, wherein an ac Wheatstone bridge having at one of its arms a variable resistance, a time constant circuit, and a controlled rectifier are all arranged in a manner that the phase of initial triggering signal is retarded to couple a load with an ac source at a relatively small phase angle; as well as disclosing that this extremely prolongs the life of an incandescent lamp.
For example, coupling of a 100 watt incandescent lamp with ac 100 volts using such switching circuit, however, has the disadvantages that the controlled rectifier, connected in series with the incandescent lamp, must be subjected to forced cooling using radiator or blower to prevent the destruction of the controlled rectifier by heat because its power consumption reaches up to 10 watts; that such forced cooling enlarges the size of the switching circuit to an unnecessary extent when a blower is used; and still that precaution may be required to reduce the noise from the blower.